Background. Ishihara Pseudoisochromatic Plates are one of the most commonly used screening tools for red-green color vision deficiencies. Even though hidden digit plates are supposed to be read only by those who are color vision defective, multiple studies report that at least some of normal trichromats can indeed read these plates. The purpose of this study was to measure chromaticity coordinates of the dots used in Ishihara Pseudoisochromatic Plates and clarify the mechanism that enables color vision defectives and some normal trichromats to read these hidden digit plates. Methods. Spectrophotometric measurements were made for the 17 numeric plates in the 24-plate version of Ishihara Pseudoisochromatic Plates (2001) using a GretagMabeth Spectrolino Spectrophotometer. The raw measurement values were used to calculate the chromaticity coordinates in the MacLeod-Boynton diagram. Results. As theoretically expected, reading of Ishihara plates by normal trichromats is mediated by the dot chromaticity differences on the L/(L+M) axis, which is the initial stage for the red-green color vision. On the other hand, reading by red-green color vision defective observers is made possible by the dot chromaticity differences on the S/(L+M) axis and luminance differences. Because hidden digit plates can be correctly read by the dot chromaticity differences on the S/(L+M) axis, normal trichromats who can extract S-cone signals effectively can read these plates that are supposed to be read only by color vision defective observers. Conclusion. Normal trichromats read Ishihara plates using their L- and M-cones. Red-green color vision defective observers rely on S-cone and luminance signals in reading the plates. Some normal trichromats can read the hidden digit plates because they can extract S-cone signal difference efficiently despite the distraction from L- and M-cones.